This Outlander article includes spoilers for the Season 5 finale and the book Breath of Snow and Ashes.
Why do so many of of the time travelers Claire meets in the past suck? First there was Geillis, who started off pretty awesome but then turned out to be a murderer and rapist, now there is Wendigo Donner, who reveals himself to be a fellow 20th century time traveler only to sit back and let Claire’s torture at the hands of Lionel Brown and the other bandits. (Not that men who are OK with and/or the active perpetrators of violence against women are solely an 18th century phenomenon.)
So who is Wendigo Donner? As is briefly touched upon in the Season 5 finale, he is another person who has traveled through the stones. Donner was part of the “Montauk Five,” a group of five men and Native American activists who traveled back from 1968. This fictional group, which is linked in the books to the real-life American Indian Movement, wanted to travel back in time to try to save their native ancestors. This group also included Robert Springer, aka Otter Tooth, who ended up with the Mohawk—Claire ran into his ghost back in Season 4. Otter Tooth was eventually banished from the tribe for the disharmony he was sowing. When he didn’t stay away, but rather returned with “a demon in his eyes,” he was killed.
In the Season 5 finale, Donner is surprised to find out from Claire that Robert Springer/Otter Tooth is dead. Though the Montauk Five were meant to all travel back in the 1760s, with the plan being to try to persuade various tribes to ally with the British forces in the Revolution, Springer had a different plan. He traveled further back in an attempt to keep the native peoples from associating with white settlers altogether. He arrived about 50 years before the rest of the Montauk Five.
In the book series, Donner doesn’t actually make an appearance until the sixth book, A Breath of Snow and Ashes, though he is mentioned in Otter Tooth’s journal, which Young Ian brings back from the Mohawk in the fifth book, The Fiery Cross. In the sixth book, Donner is part of the group that kidnaps Claire, as he is in the Outlander Season 5 finale. However, readers don’t find out he is a time traveler until later. The following year, Donner whistles “Yellow Submarine” to Brianna at Riverrun, revealing that he is a time traveler. (The Beatles connection is kept in for the TV series, as Wendigo reveals himself to Claire by asking if she knows who drummer Ringo Starr is.) It is only then that he understands he need a gemstone to travel, as Bree is the one to tell him.
We will no doubt see Wendigo Donner again in Outlander, as he is not one of the men who is killed for what they did to Claire.